1. Field of the Invention
This invention relates generally to the field of motorcycles, and in particular, to a motorcycle chassis which embodies novel techniques, geometries and configurations in order to enhance motorcycle performance, and particularly in the area of chassis, steering, and suspension performance.
2. Description of Related Art
Modern road going motorcycles are characterized by mostly evolutionary methodology and arrangements that have developed over the past forty-five years. A number of terms have been used and applied over the years of this evolution in the motorcycle art which are used in describing the related art and the inventive arrangements taught herein.
The following list of terms and their definitions will facilitate understanding the description of the invention:
An accumulator is a vessel containing compressed gas behind a piston or diaphragm. Hydraulic fluid can enter the vessel on one side of the piston or diaphragm without mixing with the gas. Gas and fluid pressure vary with changes in contained fluid volume.
Area moment of inertia is a structural property used to calculate bending stiffness and stress, a function of incremental area and the radius from its bending axis.
Bank is the slope or rotation about the axis of flight. Bank angle for a motorcycle is the angle between the tire center plane and a vertical plane through the tire contact patches.
Bump/steer is a term used to describe the tendency of a vehicle to deviate from a steered path when the suspension is suddenly compressed. Bump/steer results from interaction between steering linkage and suspension linkage, often representing design compromise and/or poor engineering.
Carbon/carbon is a composite of graphite fibers and carbon matrix, noted for its low mass density and high conductivity.
Center hub steering is motorcycle jargon that indicates one or more steering pivots located within the tire cross-section on its center plane.
The center of gravity (CG) is the apparent center point of a distributed mass. No rotation will occur in response to forces through this point of balance.
A centroid is the center of cross-sectional area.
Couple is two equal and opposite forces separated by a distance that produces (or resists) a moment.
Damping is the conversion of energy associated with mass and velocity into heat. Controlled fluid resistance or mechanical friction devices are generally used to dampen the unwanted vibrational energy associated with vertical wheel motion on vehicles.
Dive is a sudden descent. Used herein, dive refers to the tendency of a vehicle to pitch forward in response to braking forces, compressing the front suspension.
Dynamic refers to motion or laws of motion, translation and/or rotation.
Fairing generally refers to partial streamlining. Used herein, fairing refers specifically to plastic or fiberglass bodywork that comprises the complete streamlining shell for the motorcycle.
Inertia is general term for the tendency of matter to remain at rest, or to keep moving in the same direction unless acted on by a force. Inertia and mass are identical terms for linear acceleration.
Mass moment of inertia is a dynamic property which defines the resistance to rotation of a solid body about a center, a function of incremental weight in pounds, and the radius in inches from rotation axis.
Moment is the product of force times the lever arm length over which it acts.
Monocoque is a lightweight structural technique originating in the aircraft industry. Loads are carried in a stressed skin, forming the outside of the structure. The skin must form a closed box to form a stiff unit.
An orifice is a hole or reduced flow area sized to reduce flow rate or cause pressure loss at a given flow rate in hydraulic systems.
Pack down refers to the incomplete rebound of a suspension damping member, which reduce available travel for the next damping cycle.
Progressive rate generally refers to a non-linear spring rate. Ordinary springs exhibit compression distance proportional to force. Progressive wound springs require ever increasing force to produce an incremental compression.
Rising rate generally describes a suspension which produces an apparent progressive rate to wheel motion. In motorcycle jargon, a rising rate suspension refers to mechanical linkage that produces the progressive characteristic with linear springs.
A space frame is a three dimensional light-weight structure composed of tubes or solid bars that act primarily in tension and/or compression.
Squat is the tendency for a vehicle to crouch over its rear wheel in response to acceleration forces, compressing the rear suspension.
A stoppie is a reverse wheelie where the rear wheel lifts off the pavement. In this case, the deceleration moment overcomes the gravity moment.
A strut is a device which supports vehicle weight and allows compression motion, usually a shock absorber surrounded by a coil spring. Its use herein refers to a hydraulic cylinder which is pressurized from a remote accumulator.
Trail is a term which describes steering geometry. The trail is the distance the tire contact patch follows the steering axis at ground level, usually 3.0 to 5.0 inches. Less trail gives more responsive steering, but reduces stability.
A truss is a framework of tubes or struts for structural support. A space frame is a type of truss. The words space frame and truss are used interchangeably herein to indicate space frame structure.
A wheelie refers to the condition of a motorcycle accelerating with the front wheel elevated from the pavement. During a wheelie, the acceleration moment overcomes the gravity moment, achieving 100% weight transfer to the rear wheel.
In heretofore motorcycle construction, frames are primarily constructed of steel or aluminum. The tubular type frame construction utilizes steel members above and below the engine, terminating at a steering stem in front, and swing arm pivot at the rear. Multi-tube space frame structures are sometimes used for increased stiffness and lighter weight, most commonly with a single cylinder or 90 degree V-twin engines which offer less obstruction to the volume requirement imposed by such a structure.
Recent efforts have concentrated on aluminum "perimeter" frames. These use larger section members that surround the sides of the engine in a wide stance. They provide somewhat better torsional stiffness, a more direct load path between steering stem and swing arm pivot, and open access to the top and bottom of the engine.
Motorcycle steering is achieved by the use of handle bars attached directly to the steered member. Paired telescopic struts form a fork supporting the front wheel and serve double duty as the steered member and as the front suspension. Static friction in fork bushings and seals resists smooth action over bumps during heavy braking. Also, the telescopic fork arrangement, by virtue of it's length and slenderness, is relatively flexible in its resistance to brake and suspension loads, a low stiffness component in the structural chassis system. Chassis flexibility is an undesirable parameter in the search for crisp, responsive handling and stability. Moreover, fork legs and springs contribute to mass moment of inertia about the steering axis. The inertia resists rapid steering motions, contributing to rider fatigue.
Another characteristic of telescopic fork geometry is front suspension dive under hard braking. This is caused partially by weight transfer, a function of CG height, wheelbase and vehicle mass. Dive is exaggerated with the telescopic arrangement due to the necessary caster angle of the fork and steering stem. A braking force component applied along the spring/slider axis of the fork, approximately 24 degrees from vertical, results in additional compression of the front suspension, loss of ride height and (sometimes) bottoming of the suspension. Dive is accompanied by a reduction in steering caster angle and therefore stability.
To combat these difficulties, double leading link front suspension has been reborn and modernized in several recent examples such as Elf, Radd-Yamaha, Bimota and others. MacPherson types (single link/telescopic strut) have also emerged, the most successful by BMW. Most of these reduce the dive tendency and some apply anti-dive geometry to a limited degree. Steering friction increase is a practical issue with the latter types, especially under brake loads, due to a use of spherical plain bearings in the system. The spherical plain bearings are used in order to accommodate compound (steering and suspension) angular motions.
Motorcycle cornering is effected by first banking to an angle that achieves a centrifugal (radial) moment about the tire contact axis equal to the gravity (vertical) moment, at the desired cornering radius. The moment balance is maintained throughout the corner by the rider making small balance and steering corrections. With properly designed steering geometry, the motorcycle will sustain bank angle with little rider effort. The initial roll-in can be executed by rider weight shift during leisurely transition from vertical to banked.
For rapid directional change, as required in racing, the "counter-steer" technique is utilized. Counter-steer involves turning the steering member quickly in the direction opposite to the direction of turn until the desired bank angle is achieved, then back to a neutral position. Counter-steer places the front tire contact patch outboard of the vehicle CG while motion of the CG is resisted by its own inertia, producing bank angle. The ensuing gravity (vertical) moment assists the roll-in. Because the counter-steer maneuver is resisted by the entire rotational inertia of the laden vehicle mass, a rider's strength and stamina are taxed by close consecutive course bends at speed.
The design of road racing motorcycles emphasizes light weight and short wheel base to assist counter-steer for better maneuverability. Both parameters compromise straight-line stability. Reduction in wheel base increases weight transfer during hard acceleration or braking. This is evident in wheelies and stoppies, phenomena which require exceptional rider skill to deal with.
Motorcycle braking is almost always independent, front and rear wheel systems separately applied by hand and foot. Braking effectiveness, therefore is largely a function of rider sense and skill in the appropriate application of braking force to each wheel, short of wheel lockup. Anti-lock brake systems are available in some makes and models, but these systems reduce the available deceleration rate, especially on dry pavement. This is due to intermittent braking (interruption) when a wheel threatens to lock. A skilled rider can often slow or stop in a shorter distance without the anti-lock feature.
Air cooling of brakes is universal in motorcycles. Disk brakes are conventional, made of stainless steel, iron, or carbon-carbon material. Carbon-carbon is a light weight but costly material with higher temperature capability than the others. The disadvantage of carbon-carbon is frictional characteristics that vary widely over its use temperature. As disk temperature rises, stopping power increases. Much reduced stopping power is available when cold, however, and some racing organizations have ruled against the use of carbon-carbon brakes for safety reasons.
Liquid cooling of brakes has not found application in motorcycles because of cost, weight, physical size and packaging constraints, and safety concerns. Coolant must be well contained to prevent wetting the tires. Heated liquid and/or steam discharge must not be allowed proximity to the rider to prevent potential injury. However, liquid cooled brakes offer performance advantages over conventional air cooled brakes as fully described hereinbelow.
Suspension systems generally use coil springs. Sport and racing machines often employ progressive springs or rising rate spring loading geometry for tighter control of wheel travel. Damping is hydraulic, usually with adjustable (settable) damping rates. Damping force follows sinusoidal character through the wheel stroke excursion, with more rebound damping than in compression. Fixed hydraulic orifices result in instant damping force proportional to the square of stroke velocity. Insufficient damping allows bouncing to continue. Too much rebound damping causes the suspension to pack down, the incomplete rebound reducing available travel for the next cycle. Rebound damping usually employs a fluid orifice ideally sized for critical damping rate. Compression damping, however, is reduced by the addition of a pressure relief valve in parallel with a fixed orifice. This prevents excess damping force in response to extreme pavement amplitude (bumps or pot holes), allowing more suspension travel and a softer ride.
The tuning of shock absorbers is a trial and error process to find the best perceived adjustment for a set of conditions, type of riding, and or race course. Optimum damping applies only to a particular series of pavement perturbations and vehicle speed with this type of system.
Aerodynamic fairing bodywork is common to modern sport and racing motorcycles. By convention and or racing rules the fairing extends from the front axle to rear tire trailing edge. The rider is completely exposed from the side and rear view, but can be nearly hidden behind the fairing when viewed from the front. Wheels are also exposed in the side view. Wheels are open spoked structures, with tires partially covered by fenders with air cooled disk brakes to the sides on front wheels. Engine exhaust pipes run below the engine and exit through mufflers at the rear, located to the side and high for cornering clearance. The exhaust pipes are sometimes partially enclosed by the fairing. The riders hands extend beyond the fairing width, exposed to accommodate handlebar width sufficient for adequate steering leverage. Liquid cooled engines require a radiator, generally placed immediately in front of the engine. A forward facing aperture in the fairing ducts most of the through-flow air into the radiator. The warm air exits around the engine and out through side vents in the fairing forward of the riders legs, some continuing through to the rear wheel well.
The primary measurement of motorcycle performance for those skilled in the art would be achievable lap time on a closed racing circuit. Useful parameters of improved performance include: steering response and maneuverability; stability in motion; road holding/cornering ability; aerodynamic efficiency/rider wind protection; and, deceleration rate/stopping ability. All of these parameters need to be addressed to move the evolutionary development of motorcycles ahead in an uncharacteristically large step forward.